Recently, a fixing apparatus for use in an electric image forming apparatus has been developed which is disclosed in Japanese Patent Laid-Open Publication HEI 9-73243, for example, not employing a conventional heating roller type fixing system. The conventional fixing process is provided with a heat accumulation step of accumulatively heating a thin heating medium on upstream side of a fixing stage with respect to a feeding direction of the heating medium which is fed along the fixing stage, a heat/press step of clamping the heating medium and a recording medium to contact with each other, being sandwiching an unfixed toner image on the recording medium therebetween in the fixing stage after the heat accumulating step to heat and fuse the unfixed toner image on the recording medium using the heat accumulated in the heating medium as well as to press it to fix the toner image onto the recording medium, a peeling step of peeling off the fixed recording medium from the heating medium after the heat/press step. The aim to constitute such a conventional fixing process is to enable to quickly start, to correspond to a high-speed type machine, to eliminate an offset phenomenon in a case of full-color image, to obtain the color image with good quality, to reduce the electric power consumption, and to respond to a toner image having a different glossiness. Especially, in the conventional fixing process, it is disclosed that a preheat step of preheating the recording medium on the upstream side of the fixing stage with respect to the feeding direction of the recording medium is further provided.
In the conventional fixing apparatus as described above, the heating medium is constructed so as not to discharge the heat energy being accumulating the heat energy therein prior to the fixing stage, and to heat the toner image on the recording medium using the heat energy accumulated in the heating medium at the heat/press step (fixing step). Accordingly, with respect to a speed of heat transfer which is defined from the heater housed in the heating roller through the fixing belt (heat transfer belt) to the recording medium at the heat/press step (fixing step), it was difficult to accelerate a transfer speed of the recording medium which passes through the fixing stage. Even though the recording medium has been preheated prior to the fixing stage, it was also difficult to accelerate the transfer speed of the recording medium. Thus, it was difficult to accelerate the transfer speed of the recording medium to increase the copying number per a unit time (i.e., per one minute) even though the conventional technical skill disclosed in the aforesaid Laid-Open Publication was applied. As a result, it causes to set a nip width at the fixing stage to be large, whereby it was in danger of occurring the other bad influence in the fixing apparatus.
On the other hand, a fixing apparatus for an electro-photographic machine disclosed in Patent Laid-Open Publication HEI 8-2865335 is known as a preceding patent application with the same applicant as the present patent application. In the preceding patent application, a fixing belt is trained around the fixing roller and the heating roller and is constructed to preheat the unfixed toner formed on the recording sheet prior to pass a nip portion defined between the fixing roller and the pressing roller, using the heat radiation from the fixing belt which is heated by the heat conduction from the heating roller. Since the temperature of the heating roller is so controlled as to maintain the temperature thereof to a constant value, it is possible to accomplish the preheat effect certainly.
However, the atmosphere temperature in a preheat space which is defined from a space between the fixing belt and a guide plate for guiding the sheet with the unfixed toner image thereon to the nip portion and being provided under the fixing belt varies widely due to the influence of the temperature of the guide plate. Furthermore, the sheet with the unfixed toner image thereon guided on the guide plate would be preheated by a contact with the guide plate. However, the effect of the preheat from the guide plate will depend on the temperature of the guide plate. Please note that the temperature of the guide plate is different between a case just after a warming up operation is finished and a case in a continuous feeding period, depending on feed modes, as shown in a Table 1. Also, the temperature of the guide plate is different between at least a case in which the image is of mono-color and a case in which the image is of full-color, depending on the thickness (t) of the guide plate, as shown in FIG. 16. Thus, the preheat effect by the guide plate is not maintained certainly, but conversely a problem due to the bad influence of the preheat by the guide plate has been pointed out. It should be noted that the numerals indicated in the table 1 are temperature (.degree.C.) at each measuring point.
TABLE 1 mono- full-color color mode mode idle rotating time after warming up feeding feeding object to be initial just after 1 minute 3 minutes 5 minutes 7 minutes 10 minutes speed speed measured time warming up later later later later later (5 cpm) (20 cpm) Guide plate at first heating roller 28 155 158 161 161 162 162 (Thickness in fixing roller 28 155 160 163 165 166 167 1.6 mm) morning pressing rollerg 28 130 140 149 153 155 157 guide plate 28 44 58 72 77 80 83 stand-by heating roller 173 159 160 161 162 162 162 163 156 fixing roller 70 155 160 163 166 167 168 159 154 pressing roller 53 126 141 150 154 156 158 151 102.about.116 guide plate 51 56 66 78 82 84 85 64 49 Guide plate at first heating roller 26 156 159 159 159 159 161 156 155 (Thickness in fixing roller 26 148 155 158 159 161 161 154 147 0.5 mm) morning pressing roller 26 128 137 146 150 152 153 144 102 guide plate 26 74 100 109 110 112 113 79 45 stand-by heating roller 168 160 160 159 160 160 160 155 155 fixing roller 62 149 157 161 163 163 163 153 149 pressing roller 47 139 141 150 154 156 157 144 118 guide plate 58 80 112 114 115 115 116 78 48 Guide plate at first heating roller 32 157 159 161 162 162 163 157 (Thickness in fixing roller 32 156 161 164 166 167 167 154 0.2 mm) morning pressing roller 32 129 140 149 154 156 157 96.about.109 guide plate 32 98 131 138 140 141 142 48.about.52 stand-by heating roller 171 160 161 162 162 163 163 163 160 fixing roller 73 156 162 165 167 168 168 166 156 pressing roller 59 127 143 151 154 156 157 152 120 guide plate 85 102 131 140 141 142 142 104 48.about.54
On the other hand, where fixing apparatuses each of which has temperature characteristics of the heating roller, the fixing roller and the pressing roller as shown in FIGS. 4 through 6, in a case that the sheet feeding speed is changed among 40 mm/sec, 80 mm/sec and 120 mm/sec, are utilized, a glossiness of the fixed image on the sheet after the nip portion is totally different among a case just after a warm up operation in the morning (that is, from a cold condition), a case just after a warm up operation from a stand-by mode (that is, from a condition in which the temperature of the fixing roller 22 is set below 60 (.degree.C.)), and a case of a stable feeding mode (that is, just after the fixing apparatus has been actuated for 6 minutes), as shown in FIGS. 17 through 19, respectively. In other words, there is problems in which the glossiness of the fixed toner image is not stable, the quality of the fixed toner image could not be maintained to be good, when the feeding speed of the sheet is changed.
When the temperature of the heating roller is maintained in a constant value so that a predetermined glossiness would be given just after the warm up operation (that is, in a case where the temperature of the guide plate is set to be low) is finished, the excessive heat will be given to the unfixed toner and the recording medium at the stable feeding mode (that is, the temperature of the guide plate is set to be high). Accordingly, the fixing apparatus has not a fixable area in reserve with respect to the high-temperature offset. As a result, it is necessary to apply an oil onto the outer peripheral surface of the fixing belt. Therefore, it has been desired to obviate this drawback.
Accordingly, it is a primary object of the present invention to provide a fixing apparatus which is capable of ensuring the increase of the feeding speed of the recording medium, by effectively accomplishing to preheat the recording medium.
It is another object of the present invention to provide a fixing apparatus which is capable of enhancing the preheat effect by preheating uniformly the recording medium.
It is different object of the present invention to provide a fixing apparatus which is capable of maintaining the good quality of the toner image even though the feeding speed of the recording medium is increased.